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J. H. Yoon, H. A. Swarbrick; Posterior Corneal Shape Changes With Overnight Corneal Edema in Rigid Conventional and Orthokeratology Lens Wear. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4848.
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To evaluate the influence of overnight corneal edema on posterior corneal shape during sleep, and conventional and orthokeratology (OK) rigid gas-permeable (RGP) lens wear.
Eighteen young adult subjects (19-32 yrs) with low myopia (<-4.00D) and astigmatism (<1.50D) were fitted with reverse geometry OK lenses (BE; Capricornia Contact Lens, Brisbane, Australia) in Boston XO material (nominal Dk/t 46). Lenses were worn overnight only for 14 days. A separate group of 10 subjects (19-32 yrs) wore J-Contour conventional RGP lenses (Capricornia Contact Lens) in Boston XO material (nominal Dk/t 56) for one night in one eye only. Posterior corneal apical radius of curvature (Ro; mm) and asphericity Q were calculated using an in-house program based on the anterior corneal ellipsoid curve measured using the Medmont E300 topographer (Medmont, Camberwell, Australia) and corneal thickness measured with the Holden-Payor optical pachometer across the horizontal meridian. Data were collected at baseline and within 1 hr of eye opening. Repeated measures ANOVA with planned comparisons was used to compare changes from baseline, with a critical p-value of 0.05.
In the non-lens wearing eye there were no changes in posterior Ro or Q after sleep. In the conventional RGP lens-wearing eyes, there was significant flattening of posterior Ro (p=0.001) and significant increases (towards oblate) in posterior Q (p=0.001) after overnight lens wear. In the OK lens-wearing eyes, myopia reduced from -2.64 ± 0.99D (mean ± SD) to -0.39 ± 0.49D over 14 days of overnight OK. There was significant steepening of posterior Ro in the morning on days 7 and 14 (p<0.001). Moreover, there were significant increases in posterior Q on days 1 (p<0.001) and 4 (p=0.011). In contrast, there was a significant decrease in posterior Q on day 14 (p=0.013).
The findings of this study are consistent with previous research demonstrating inhibition of central edema in overnight OK, but a normal edema response in the mid-periphery. Analysis relative to a fixed 8mm chord demonstrates that the cornea swells in a posterior direction with overnight edema.
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